In general, an "elastomeric material" or "elastomer" includes any material which can be stretched, bent, compressed, or otherwise distorted when subjected to force, and which is capable of returning to substantially the undistorted shape within a reasonably short time after the force is removed. An elastomer must be capable of a substantial degree of stretching under tension before breaking. Elastomers are useful in a wide variety of applications, including fluid-impervious seals between mechanical parts, seals between moving parts, electrical insulation, and various forms of non-rigid coupling of mechanical parts such as shock absorption and vibration damping.
Elastomers may be subjected to a wide variety of adverse or hostile conditions or environments. For example, elastomers may be exposed to high temperatures, high pressures, corrosive or abrasive substances, and solvents. To withstand such conditions, it is highly desirable to have elastomeric materials which can withstand hostile environments for substantial periods of time. Such materials may be useful in geothermal operations, oil and gas drilling and production operations, chemical processing and handling operations, power generation and handling, and a wide variety of other uses.
In the search for non-metallic, non-ceramic materials that can withstand hostile environments, substantial attention has focused upon fluorocarbon polymers. As used herein, the term "fluorocarbon" comprises molecules that contain both fluorine and carbon, regardless of whether such molecules also contain other atoms such as hydrogen, oxygen, or chlorine. Carbon-carbon bonds and carbon-fluorine bonds are relatively strong and stable, and a variety of polymerization reactions [1] and fluorination reactions [2] are well known in the art.
Polymerization reactions may be performed in a variety of sequences, utilizing a variety of starting materials which are usually called "monomers." Unsaturated monomers such as ethylene or propylene are commonly used because they contain double bonds between adjacent carbon atoms which provide useful, easily controlled reaction sites. For example, ethylene and propylene may be reacted to provide a polymer as follows: ##STR1## where "n" is a large number. Using the conventional terminology, this polymer may be named after its starting materials e.g., "ethylene-propylene" or "copolyethylene-propylene," even though there are few, if any, unsaturated bonds left in the material after it is polymerized. The prefix "co-" in "copoly-" indicates that two or more monomers are used as starting materials. Different monomers may be incorporated into a copolymer in random sequence, alternating sequence, block sequences (e.g., long stretches of "A" monomer alternating randomly with long stretches of "B" monomer), or graft form (e.g., side chains of "A" monomer on a backbone of "B" monomer).
It is possible to control the quantity or molar ratio of monomers that are present during a polymerization reaction. For example, if 4 moles of ethylene and 1 mole of propylene are mixed and polymerized, the resulting chain may be represented as either of the following: ##STR2## This molecule is, in effect, a long saturated chain of carbon atoms called a "backbone," with one methyl group (--CH.sub.3) attached as a "moiety" on approximately every tenth carbon atom on the average. Other feedstocks may be used to create other types of moieties attached to backbone chains.
In general, a moiety is a molecular group that is attached to a backbone chain in a branch or pendant configuration. For example, a methyl, ethyl, vinyl, propyl, butyl, aryl, methoxy, or other organic group that is covalently bonded to a long carbon chain is a moiety. By contrast, a single atom such as hydrogen, chlorine, fluorine or oxygen which is attached to a backbone chain is not regarded herein as a moiety. In addition, if two polymeric backbone chains are crosslinked to each other, then neither is regarded as a moiety of the other molecule.
Moieties play a very important role in the physical characteristic of polymers, by preventing backbone chains from fitting closely against other backbone chains. Polymers that do not have moieties tend to be regularly shaped, symmetric molecules that can fit closely against similar molecules. Such molecules tend to solidify or "crystallize." In comparison, polymers that have numerous moieties tend to be less able to crystallize, and therefore tend to remain in a fluid state.
A variety of techniques are known which are capable of "crosslinking" polymeric molecules, i.e., creating bonds between backbone carbon chains. Such bonds may be between carbon atoms of backbone chains, or they may be through oxygen, nitrogen or sulfur atoms, a variety of moieties, or other non-backbone atoms. Techniques for crosslinking polymers include electron radiation, gamma radiation, ultraviolet radiation, the use of substances such as peroxides which create free radicals, and the use of chemically reactive substances such as diamines [3]. In the manufacture of elastomers with high performance characteristics, crosslinking bonds are usually covalent. However, it is possible to create crosslinked materials that are suitable for some purposes which contain non-covalent crosslinking bonds.
Fluorine can be incorporated into a fluorocarbon polymer in a variety of ways, including the following:
1. fluorine may be present in one or more of the monomers that are polymerized. For example, three illustrative reactions are: ##STR3## 2. fluorine may be added or substituted for hydrogen by "direct" fluorination of a hydrocarbon polymer or a partially fluorinated polymer [4]. The phrase "direct fluorination" implies that elemental fluorine, as fluorine gas (F.sub.2), is contacted with the material being fluorinated. For example, two such reactions (in non-stoichiometric form) are: ##STR4## PA1 3. fluorine may be added indirectly to a polymer. The word "indirectly" implies that the fluorine is supplied in the form of a compound, such as molybdenum hexafluoride (MoF.sub.6) or cobalt trifluoride (CoF.sub.3), which releases fluorine when heated or otherwise manipulated. PA1 a. the polymeric backbone chains must have a suitable number of moieties attached; PA1 b. partial fluorination, which perferably should occur by incorporation of one or more fluorine-containing monomers in the polymerization reaction. The degree of partial fluorination must be sufficiently low to allow satisfactory crosslinking, yet sufficiently high to prevent excessive breakage of backbone chains during the perfluorination reaction; PA1 c. the partially fluorinated polymeric backbone chains must be crosslinked to a desired degree by means which did not cause excessive breakage of backbone chains. PA1 (1) Thermogravimetric analysis. In this process, a sample is placed in an inert atomosphere, and heated at a known rate, such as 10.degree. C. per minute. The sample is continuously or intermittently weighed, to determine the amount of weight loss as a function of increasing temperature. PA1 (2) Swelling in solvent. Halogenated organic solvents, such as trichlorotrifluoroethane, are capable of dissolving uncrosslinked polymeric molecules and diffusing into crosslinked molecules causing swelling. A highly crosslinked elastomer will not swell as extensively as a lightly crosslinked elastomer. To evaluate swelling, a sample of elastomeric material is immersed in solvent, and at periodic intervals is removed, quickly blotted, and weighed or otherwise measured. This provides an indication of the amount of crosslinking in the sample. PA1 (3) spectroscopic analysis, using infrared or other wavelengths. Different types of chemical bonds absorb infra-red radiation of different wavelengths. For example, carbon-hydrogen bonds tend to absorb infra-red light with relatively short wavelengths, while carbon-fluorine bonds tend to absorb infra-red light with somewhat longer wavelengths. By passing a beam of infra-red light through a thin sample of material, and analyzing the absorption of light at various wavelengths, it is possible to obtain information about various types of bonds within the sample. PA1 (4) Strength and elasticity tests. A highly crosslinked elastomer tends to be stiff and strong; a lightly crosslinked elastomer tends to be soft and flexible. Mechanical tests are available which measure several types of deflection and deformation, such as the amount of elongation before breakage under tension, the amount of tension required to break a sample, and the amount of compression as a function of pressure. Such tests may be performed by calibrated equipment using samples with controlled crossectional areas, or by simply handling the material being examined. PA1 (5) Degradation testing. In this method, a sample of the material is immersed in hot brine, solvent, corrosive fluids or gases, or other adverse conditions. The sample is removed, if necessary, and examined periodically to determine its rate of deterioration. Such examination may include the tests listed above, although spectroscopic analysis may be limited by discoloration of the sample.
F.sub.2 reacts very rapidly and exothermically when contacted with hydrocarbons. In order to slow down the reaction and prevent explosions, combustions, or undesired byproducts, direct fluorination may be commenced at low temperature or low pressure, and in the presence of an inert gas such as helium.